A spark plug is used to ignite an internal combustion engine such as a gasoline engine. The spark plug generally includes a center electrode, an insulator disposed at an outer side of the center electrode, a metal shell disposed at an outer side of the insulator, and a ground electrode. The ground electrode is installed on the metal shell and forms a spark discharge gap between the ground electrode itself and the center electrode.
A spark plug as described above is disclosed in, for example, Japanese Patent Application Laid-Open No. 6-196247. This spark plug includes a leg base portion at the insulator. The leg base portion faces a step portion formed on the metal shell with a clearance therebetween. The leg base portion is formed approximately parallel to the axis line of the spark plug. This leg base portion inhibits a combustion gas from entering between the insulator and the metal shell so as to reduce variation in heat resistance.